JC polyomavirus (JCV) infection in humans can cause a demyelinating disease of the central nervous system, progressive multifocal leukoencephalopathy (PML). However, JCV infection usually does not result in PML in healthy subjects. JCV infection is prevalent in many human populations without causing widespread PML. PML typically only develops in JCV-infected subjects that also have a weakened immune system. Subjects that are immuno-compromised due to a disease or an immunosuppressive treatment may be vulnerable to PML associated with a JCV infection.